Modulated microwave source

ABSTRACT

A microwave modulator comprises a microwave resonant cavity having a relatively rigid wall and dimensions which establish a microwave resonant frequency for the cavity, means for introducing energy at microwave frequencies to the cavity, an aperture formed in a cavity wall, and a wall member comprising a relatively thin, flexible body which extends continuously across the aperture and presents an electrically conductive surface to microwave energy. The body is positioned for deformation by an external exciting force which is applied to an externally exposed surface of the body. The exciting force which deforms the flexible body causes a variation in the physical dimensions and in the resonant frequency of the cavity. The resonant frequency pre-established by the dimensions of the resonant cavity are therefore varied in accordance with the variations in the deformation of the body and microwave energy at the predetermined resonant frequency is modulated accordingly.

I451 May'28,1974

mired States Patent 1191 Qohen ABSTRACT [5 1 MODULATEDMKIROWAVESQURCE lnventor: Leonard 1). Cohen, Brooklyn, NY.

. v, v I v A microwave modulator comprises a microwave reso- [73] Assrgnee: GTE Laboratories Incorporated,

nant cavity having a relatively rigid wall and dimensions which establish a microwave resonant frequency for the cavity, means for introducing energy at micro- Waltham, Mass.

quencies to the cavity, an aperture formed in and a wall member comprising a relah-extends continuously erture and presents an electrically conductive surface to microwave energy. The body is positioned for deformation by an external exciting force which is applied to an externally exposed surface of .w h W V. d O b k m M L, p m a w m m e nv m e yS V d aC r C wafla 60 ZON 1 7 1 51 Z D/ 3 2 3 ,3 "H52 25 m m 6 u 2R m E u 5 7 mm 9 1 n 8 L5 ""2 3 5 NW1 A mmny w nm mTm2 A l mmr u e M u S f. O l C e p s l n P mw F A UIF U M N n m 555 the body. The exciting force which deforms the flexi- [56] References Cited UNITED STATES PATENTS Fredholm et al. Reed,

microwave energy at the predetermined resonant frequency is modulated accordingly.

Primary Examiner-Robert L. Griffin Assistant Exgminer-Aristotelis M. Psitos ffiirawing Figures Xit br n ey, A gent Robert A. Walsh or Firm-Irving M. Kriegsman? sli I silk- PATENTEMY 28 m4 SHEEIINZ V/IR/IIBL E PRESSURE CIT/7770A SHEEI 2 BF 2 l MODULATED MICROWAVE SOURCE This invention relates generally to microwave energy sources and more'particularly to an improved means for modulating a source of microwave energy.

in various applications, it is desirable to provide a relatively compact, inexpensive, rugged and 'portable means for transmitting intelligence at microwave frequencies. Such applications include for example the transmission of pressure and temperature variations from a number of remote stations to a control station, and, the transmission of secure voice communications.

An important factor in providing microwave apparatus capable of satisfying this need is the modulation of an energy source operating at microwave frequencies. Various proposals have been made for modulating a microwave energy source. Generally speaking, a microwave energy source includes a-means capable of generating oscillations at microwave frequencies'and a resonant cavity which in cooperation with this means functions to establish a desired frequency of oscillation for the energy source. The resonant cavity comprises a confined walled conduit whose cross-sectional configuration and dimensions determine the mode' and frequency of resonance.

A method of microwave modulation is, known wherein the wall dimensions of a resonant cavity are altered in accordance with modulation intelligence. In one arrangement, an end wall is formed by an elongated rod or piston which is displaced within a bore or cylinder of the cavity under the control of a bellows to which variable pressures are applied. In another known arrangement, an electromechanical transducer supporting a plurality of circular surfaces which are concentrically positioned in a plane forms a terminating wall segment of a resonant cavity. The circular conductive surfaces are insulated from each other and these surfaces are displaced in unison by either electrical or mechanical means thereby resulting in'frequency modulation of the microwave energy source.

The aforementioned means for altering the dimensions of a resonant cavity in order to provide modulation of the microwave source are relatively complex in structure, are not sufficiently rugged for portable use, generally require additional means for actuating the wall member, and limit the modulation sensitivity and tuning range of the oscillator. More particularly,'with respect to the later limitation, the use of a plurality of circular conductive bodies as a terminating wall member with a cylindrical shaped resonant cavity limits the operating mode of the microwave source to a TE mode since there are noradial currents and the electric field is circular. The modulation sensitivity and tuning range of the microwave source is thus more limited than would be the case with a rectangular crosssectional configuration for a resonant cavity.

Accordingly, it is an object of the present invention to provide an improved modulatedmicrowave source.

Another object of the invention is to provide a modulated microwave source having increased modulation sensitivity and tuning range.

Another object of the invention is to provide a modulated microwave source capable of operating in a TE mode of oscillation in cylindricalgeometry and in the TE mode of oscillation in rectangular geometry.

Another object of the invention'is to, provide a relatively noncomplex and rugged modulated microwave transmitter adapted for portable use.

A further object of the invention is to provide a relatively noncomplex modulated microwave transmitter adapted for operation as a remote sensor.

Another object of the invention is to provide an improved voice modulated microwave source.

Another object of the invention is to provide an improved means for directly altering the physical dimensions and resonant frequency of a resonant cavity with mechanical excitation energy directed at a wall memv ber of the cavity.

Another object of the invention is to provide an improved means for frequency modulating a microwave source.

A further object of the invention is to provide an improved means for amplitude modulating a microwave source. I

Still another object of the invention is to provide a relatively noncomplex and portable transceiver which operates at microwave frequencies. I

In accordance'with the general features of this invention, a microwave modulator comprises 'a microwave resonant cavity having a relatively rigid wall and dimensions which establish a'microwave resonant frequency for the cavity, means for introducing energy at microwave frequencies to the cavity, an aperture formed in a cavity wall, and a wall member comprising a relatively thin, flexible body which extends continuously across the aperture and presents anelectrically conductive surface to microwave energy. The body is positioned for deformation by an external exciting force which is applied to an externally exposed surface of the body. The exciting force which deforms the flexible body causesa variation in the physical dimensions and in the resonant frequency of the cavity. The resonant frequency pre-established by the dimensions of the resonant cavity are therefore varied in'accordance with thevariations in the deformation of the body and microwave energy at the predetermined resonant frequency ismodulated accordingly. An exciting force comprises for example a force established upon the wall member by acoustical variations in air pressure created by the voice of an individual speaking toward the wall member. Other alternative excitations applied directly to the flexible body are derived from a source whose pressure, temperature or other characteristic is being monitored.

A transceiving apparatus in accordance with a feature of the present invention comprises a resonant cavity having a relatively rigid wall and dimensioned to establish a predetermined resonant cavity frequency, means for introducing microwave energy into the cavity, an aperture in the wall, and a wall member formed by a relatively thin flexible body extending continuously across the aperture and presenting an electrically conductive surface to microwave energy. The wall member is positioned in a manner for deflection by an external activating force. The transceiver further includes antenna means for radiating modulated microwave output energy and a means for selectively detuning the microwave. cavity by a predetermined frequency Af for causing a shift in the frequency of the microwave oscillations from the first predetermined frequency f, to a second frequency f A signal of frequencyfi which is received by a transceiver mixes with the locally generated microwave frequency f to provide an intermediate frequency. Signal detection and transducing means are also provided.

These and other objects and features of the invention will become apparent with reference to the following specification and to the drawings wherein:

FIG. 1 is a schematic view of one embodiment of a modulated microwave source constructed in accordance with features of this invention;

FIG. 2 is a sectional side view of a modulated microwave transmitted constructed in accordance with features of this invention;

FIG. 3 is a front view of the transmitter of FIG. 2;

FIG. 4 is a view illustrating an alternative means for exciting a' deformable wall member-of a resonant cavity of the transmitter of FIG. 2;

FIG. 5 is a schematic view of two transceivers constructed in accordance with features of this invention, one of which is operating in a transmitting mode the other of which is operating in a receiving mode;

FIG. 6 is a sectional view illustrating a modification to the transmitter of FIG. 2 for shifting the frequency of the transmitter;

FIG. 7 illustrates the frequency deviation caused by the tuning means of FIG. 6; 1

FIG. 8 is a schematic view of an amplitude modulating microwave source constructed in accordance with features of this invention;

FIG. 9 is a perspective view of a rectangular waveguide cavity useful in examining the mathematical relationships of a microwave source; and,

FIG. 10 is a sectional view of an alternative mounting arrangement for a diaphragm utilized with a modulating source of this invention.

Referring now to the drawings, a schematic diagram of a microwave source 10 which is frequency modulated in accordance with features of the present invention is illustrated in FIG. 1. The microwave source includes a resonant microwave cavity referenced by numeral l2 and a negative resistance element 14 positioned within the cavity. Proper biasing of the negative resistance elements 14 will result in a dc. to r-f energy conversion and a sustained r-f oscillation will exist in the cavity. The frequency of oscillation is determined by the resonant frequency of the cavity. As is well known, the physical dimensions of the cavity are selected in order to establish a resonant frequency f of the cavity. The negative resistance device can comprise any of the well known solid state devices which exhibit a negative resistance over a range of microwave frequencies. Such devices include Gunn diodes and avalanche diodes. Operating potential is provided for the negative resistance diode element 14 by a battery 16 which is coupled to one electrode of the diode through a lead-in wire 18 and a ground circuit. Microwave output energy from the microwave source 10 is coupled from an output port 20 of the cavity.

The microwave cavity includes as an integral wall member thereof a relatively thin, flexible, diaphragm body 22 which is positioned on the microwave cavity 10 in a manner for exposing the diaphragm toexcitation from an external source. The diaphragm 22 is fabricated of a material which presents to the microwave energy a continuous electrically conductive surface. The diaphragm 22 may be fabricated of a metal such as nickel, or any aluminum alloy such as Dural metal, or other suitable metal or alternatively the diaphragm 22 may be fabricated of an electrically insulating material such as Mylar which is coated with an electrically conductive material. Because the diaphragm 22 is an integral wall member of the resonant cavity, displacement of the diaphragm from an undeflected position will cause a variation in the cavity dimensions'and a corresponding variation in the frequency of oscillation from the established frequency f A static or dynamic displacement of the diaphragm 22 will therefore change the resonant frequency of the cavity and hence the frequency of oscillation. The diaphragm thus functions as a modulator element transforming mechanical displacement into frequency modulation of the microwave source. The mechanical excitation of the diaphragm can be derived from acoustic, thermal, magnetic or kinetic energy sources. The modulated microwave source of FIG. 1 therefore provides a simple, inexpensive and efficient method to modulate at high or low carrier levels. The modulator advantageously has zero insertion loss and it requires no electrical power. In addition, because of its relative simplicity, it is reliable, reproducible and substantially free of A.M. distortion.

For a rectangular waveguide cavity, shown in FIG. 9, the frequencies of resonance as a function of cavity dimensions are given by:

where fis the resonant frequency, v is the velocity of light, and l, m, and n are the number of halfwavelengths in the directions indicated. The rate of change of the resonant frequency of the cavity with cavity length for a given mode of resonance is given by:

afldC -vn /2C [(lC/A) (mC/B) n ll 2 For small changes in cavity length, the variation'in resonant frequency with cavity length as the diaphragm is displaced will be essentially linear. In addition, a low level mechanical excitation of the diaphragm will produce a large frequency deviation of the carrier and hence a high index of modulation. As obtained from Eq. (2) for a TE mode of resonance at 10 GI-Iz in an X-band rectangular waveguide cavity (A 0.900 in., B 0.400 in.), a frequency deviation of approximately 7 MHz will result from a 0.001 in. planar displacement of one end wall. Since small changes in resonant frequency produce negligible changes in the Q of the cavity, amplitude variation in r-f output level due to changes in cavity Q will be negligible.

In one embodiment of the present invention illustrated in FIGS. 2 and 3, a frequency modulated microwave transmitter or remote sensor is illustrated. The transmitter includes a resonant cavity which is formed by the body 30, aflexible metallic diaphragm 32, and an antenna support and mount body 34. The cavity diaphragm comprises a rectangular cavity extending between an end walled member formed by an inner surface 36 of the body 34 at one end and by the diaphragm 32 at an opposite end. An adjustable tuning screw 37 is provided and engages a threaded aperture of the wall of the cavity 30 for tuning the cavity over a range of frequencies. The diaphragm extends over the rectangular cross-section of the cavity and is mounted in place between a flange segment 38 of the body 30 and a clamping body 40 which aligns with the flange segment 38. The clamping body 40 includes an aperture 42 which provides for acoustical excitation of the diaphragm 32 by acoustical energy impinging on an outer surface 44 of the diaphragm. The diaphragm wall member will therefore flex thereby altering the dimensions of the resonant cavity in accordance with the frequency cludes a threaded segment thereof which is adapted to engage an internally threaded wall segment 52 of an integral cylindrically shaped mount segment 54 which extends from the cavity body 30. A similar oppositely disposed cylindrically shaped mount body 56 extends from the cavity body 30 and includes a bore for receiving an electrical insulating sleeve 58 and a conductive rod 60 formed of silver plated brass. The'rod 60 is maintained in electrical contact with the center conductor of a coaxial connector 62. Electrical operating potential is derived from a battery 64 and is coupled to the connector 62 via a mating connector 66. A positive terminal of the battery 64 is therefore conductively coupled to one electrode of the diode 46 through the connectors 66 and 62 and through the conductive rod 60. The insulating sleeve'58 which serves to insulate the conductive rod 60 from the body 56 also functions as an r-f bypass for r-f currents which could undesirably be radiated by the power source leads. A negative terminal of the battery 64 is coupled to an opposite electrode of the diode 46 via a ground circuit connection.

The antenna mount 34 supports an antenna coupler 70 having a conductive pickup rod 74 extending through the body 34 into the resonant cavity. The rod 72 is electrically insulated from the surrounding conductive material of the body 34 by an insulating sleeve 74. An antenna 76 is mounted to the coupler 70 and microwave energy which is coupled to the rod 72 will be radiated by the antenna body 78. The antenna 78 is selected to exhibit gain and highly directional characteristics in order to provide in one application a security voice communication system or in an alternative application a remote sensor for use with a base station. The antenna 78 is selected to exhibit omnidirectional characteristics in other applications requiring this form of radiation.

The diaphragm 32 of FIGS. 2 and 3 is shown to be rigidly mounted as a wall member of the resonant cavity 3t). Excitation of the diaphragm will cause displacement of the diaphragm and substantially undistorted modulation will occur for relatively small displacements of the diaphragm. This will occur for example when the diaphragm is used for direct voice modulation. However, in some applications the displacement of the diaphragm may be relatively large and it is then preferable that the displacement of the diaphragm wall member extending across the cavity aperture occur as a uniform displacement of the entire planar body in order to provide distortionless modulation. An altemative embodiment of the invention provides for the mounting of the diaphragm as a wall member and for providing for displacement of the diaphragm as planar body. This is accomplished as illustrated in FIG. in accordance with another embodiment of the invention by providing a flexible mounting means for the diaphragm. FIG. 10 illustrates a bellows mounting means 79 operating as a flexible mount. The bellows, which is formed of metal or other suitable material, is mounted to the walls of the cavity 30 at an outer portion thereof at the aperture in the wall. The diaphragm 32 is secured at an inward end of the bellows. Exciting energy which is incident on the diaphragm causes relatively long displacements and the entire surface of diaphragm will be displaced equally in distance as a planar surface and modulation distortions will be substantiallyreduced.

The modulator microwave source in accordance with the present invention is adapted for directed excitation of a relatively thin, flexible mechanical wall member by acoustic, thermal, magnetic or kinetic energy sources. FIGS. 2 and 3 illustrated an arrangement adapted for exciting the diaphragm 32 with acoustical energy. FIG. 4 illustrates an alternative embodiment of the invention wherein the diaphragm is excited by a variation in pressure applied to the diaphragm from a source 80. A pressure from the source which may comprise a variation in the pressure exerted by a liquid or a gas is applied to an outer surface 44 of the diaphragm through a conduit end mount 82 which is suitably bonded to and provides a pressure seal with the diaphragm 32 and which is mounted to the flange 38.

A transceiver communication system constructed in accordance with features of the invention is schematically illustrated in FIG. 5. Those elements of FIG. 5 which perform functions similar to the elements of FIG. I. bear the same reference numerals. The transceiver arrangement of FIG. 5 includes a first transceiver unit which is shown to be radiating a modulated signal of frequency 1, and a second transceiver unit 92 which is shown to be receiving this "signal. Each of the transceivers provides for voice actuated frequency modulation of the microwave frequency f, and is constructed in accordance with the structural features illustrated in FIGS. 2 and 3. When operated in a transmit mode, either of the units 90 and 92 will transmit a frequency modulated signal at a frequency )1. However, during a receiving mode of operation, the receiver is modified in order to shift the resonant cavity frequency and correspondingly the frequency of oscillation from that of the transmit frequency f, to a different frequency f, as shown in FIG. 7. Although FIG. 7 illustrates the frequency of the receiver as being reduced in frequency, it may alternatively be shifted for an increase in frequency above the transmitting frequency. A frequency modulated signal which is transmitted by the unit 90 and received by the unit 92 will experience some f-m to a-m conversion since the resonant frequency of the cavity of unit 92 is detuned with respect to the transmit frequency f,. In addition, the received signal will be mixed by the oscillating diode in well known manner. Therefore, an amplitude modulated signal at an intermediate frequency f, will be generated and will exist on the line 18 which couples the battery to the diode 46. This intermediate frequency which will be in the megacycle frequency range is coupled from the line 18 by conventional means, such as tuned circuits, to an a-m detector where the modulation components are extracted, amplified and reproduced by a transducer 94. Similarly, when the transceiver 92 is transmitting, it will transmit at a requency fi while the transceiver unit 90 will be detuned to a frequency f,. An a-m detector and audio amplifier unit 96 as well as a transducer 98 as was described with respect to the unit 92 of FIG. is also provided for the unit 90.

A means for tuning the transceiver during the receiving mode in order to shift the frequency of oscillation from a frequency f, to a lower frequency f,, for example, is illustrated in FIG. 6. FIG. 6 is an enlarged partial view of the resonant cavity wall segment of FIG. 2 extending between the flange 38 and the support member 54 and modified to provide the desired tuning. The wall includes an aperture 100 formed therein and a cylindrically shaped plug 102 formed of electrically conductive material positioned in the aperture 100. The plug 102 is mechanically biased by a spring 104 for establishing that a lower surface 106 of the plug 102 is normally flush with a lower surface 108 of the resonant cavity body 30. The spring 104 is positioned between an upper shoulder 110 of the cavity wall and a cap 112 which is press fitted to the plug 102. By exerting a force on the cap 112 such as a user might readily accomplish by finger pressure on the cap, the plug descends into the cavity for a distance determined by the position of the cap 102, the length of the plug and the position of the upper surface of the body which functions as a stop. In this depressed condition, the presence of the plug in the resonant cavity causes tuning of the cavity to a frequency f, and the microwave oscillation frequency is altered to the frequency f,. As finger pressure on the cap 112 is released, the plug 102 will withdraw under the influence of the spring force to a stop position provided by a lower shoulder 114 which engages a lower shoulder 116 on the plug 102. Thus, a relatively simple and noncomplex arrangement is provided-for converting from a transmit to a receive mode of operation. In an alternative embodiment of the transceiver, a user can continuously monitor a transmissionby modifying the resonant cavity dimensions for providing that the transceiver is tuned for reception when the plug 102 is withdrawn and tuned for transmitting when the plug is depressed. A modulated transmission is then provided by depressing the plug 102 and deflecting the flexible member 22.

While amplitude modulated signal processing was described with respect to the reception and detection of intelligence in the transceiver of FIG. 5, a frequency modulated intermediate frequency signal is also generated. This frequency modulated signal can then be converted and detected with well known frequency modulation circuit techniques to provide f-m detection of the audio modulation. The tuning plug 102 is used in the same manner for frequency shifting the receiver as previously described.

The transceiver of FIG. 5 derives electrical operating energy from the battery source 16 which also provides operating potential for the negative resistance oscillating element 14. The antenna 76, since operating in the microwave frequency range, can comprise a relatively simple antenna of small size which is designed for high gain and directivity. The transceiver described affords a secure communication system in a simple, compact form with a mimimum number of components.

Modulation of a microwave source in accordance with another embodiment of the invention can be accomplished by amplitude modulation. A schematic representation of an amplitude modulated microwave source is illustrated in FIG. 8. Those elements of FIG. 8 which perform a function similar to that performed by similar elements in FIG. 1 bear the same reference numerals. In the embodiment illustrated in FIG. 8, the diaphragm 22 comprises a wall segment of a transmission type modulating resonant cavity 118. This cavity is resonant at a frequency f, while microwave carrier energy at a frequency fl differing from f: and which is generated by an external microwave source is coupled to the modulating cavity. Excitation of a diaphragm 22 will cause mechanical displacement of the cavity diaphragm thereby changing the resonant frequencyf of the cavity 118. As the resonant frequency of the cavity 118 varies from f the insertion loss of the cavity to the fixed frequency carrier f, will also vary. This will result in amplitude modulation of the microwave carrier. The transmission type of resonant caVity 118 is constructed in accordance with the general configuration of FIG. 2. However, the negative resistance diode element 46 and supporting structure is eliminated and external means are provided for generating microwave energy at the frequency 1", and for coupling into the cavity energy from the external source 120.

A" solid state frequency modulated transmitter of the type described and illustrated with respect to FIG. 2 and 3 was constructed and successfully operated at 10 6112. A Gunn diode was employed as a microwave energizing. source in a rectangular type cavity. The diaphragm comprised a flexible metallic diaphragm of 0.0005 inches thickness'and was used as'one end wall of the cavity. It was found that a voice directed at the diaphragm mechanically modulated the diaphragm which in turn produced a frequency modulated microwave signal at the'output port of the cavity. This signal was transmitted to a receiver, was demodulatedand was audibly reproduced. The reproduced audible signal was free of any perceptible distortion. In addition, the modulator was sufficiently sensitiveto permit the voice source to .be remotely located and yet effectively energize the diaphragm. In effect, the transmitter functioned as a microwave microphone.

The present invention finds use in various applications. Because the diaphragm type modulator can be activated by temperature or pressure changes, the transmitter of FIGS. 1, 2 and 3 can be used as a remote, wireless indicator in control and monitoring applications. Existing temperature, pressure, flow or liquid level switches, for example, can be altered in order to cause the actuating mechanism of the switch to operate the diaphragm of the microwave cavity. In such industries as oil refining or chemical processing where the plant may extend over a large area, a wireless type indicator transmitting to a central process control center affords a fast and inexpensive means for process control and monitoring.

An improved microwave modulator has thus been described which is relatively noncomplex, rugged, portable and is adapted to operate both in a TE mode in cylindrical-geometry or the TB mode in rectangular geometry, thereby enhancing modulation sensitivity and tuning range.

While I have described particular embodiments and features of my invention, it will be apparent to those skilled in the art that various modifications may be made therein without departing from the spirit of the invention and the scope of the appended claims.

What is claimed is:

1. A modulated microwave source having an output comprising:

by an external force, said end wall is mounted by a flexible bellows mounting means thereby providing uniform displacement of the flexible end wall responsive to excitation by energy incident thereon, the movement of said end wall changes the dimension of said cavity which changes the oscillating frequency of said cavity whereby the output is frequency modulated. 

1. A modulated microwave source having an output comprising: means including a resonant cavity and a microwave oscillating device located within and electrically connected to said cavity for sustaining oscillations at a frequency in a microwave frequency range; said resonant cavity having a thin, flexible end wall having a continuous surface and presenting an electrically conductive surface to microwave energy within said resonant cavity, said end wall positioned in a manner for exposure to and excitation by an external force, said end wall is mounted by a flexible bellows mounting means thereby providing uniform displacement of the flexible end wall responsive to excitation by energy incident thereon, the movement of said end wall changes the dimension of said cavity which changes the oscillating frequency of said cavity whereby the output is frequency modulated. 